1. Field of the Invention
The present invention relates to a multi-section vacuum pump including a drive shaft having a shaft seal structure on the drive shaft. The multi-section vacuum pump according to the present invention is applicable to a multi-section dry type vacuum pump which is operated at a high compression ratio in the range of a suction pressure as low as 10.sup.-3 Torr at a relatively high temperature.
2. Description of the Related Arts
In general, in a dry type vacuum pump, a leakage of air from without to within the pump must be as small as possible, to realize a pump having a high performance, and when pumping a combustible or corrosive gas, it is particularly important from the viewpoints of safety and corrosion-resistibility to minimize the intrusion of oxygen and moisture contained in the atmosphere inside the pump. In a prior art multi-section dry-sealed vacuum pump which is operated, in particular, at a high compression ratio and at a relatively high operating temperature due to the compression heat, since the pump is driven by a motor or the like installed outside the pump, a drive shaft must be passed through the housing of the pump to the outside of the pump, and accordingly, a shaft seal structure must be mounted on a drive shaft protruding portion to seal same.
In a three-section vacuum pump as shown in FIGS. 8 and 9, a portion where a drive shaft 101 passing outward through the housing of the pump is provided with a lubricating oil reservoir 106 accommodating a combination of a fixed ring 103 and a rotary ring 104, in which the fixed ring 103 of a bearing is secured inside the lubricating oil reservoir 106, the rotary ring 104 slidingly rotates together with the drive shaft on an inner surface of the fixed ring 103, and the thrust surface between the fixed ring 103 and the rotary ring 104 is supplied with lubricating oil reserved at and splashed from the lubricating oil reservoir 106 by a splasher 105 attached to a driven shaft 102 which rotates in a reverse direction to that of the drive shaft 101 while driven thereby via timing gear set 109. Further, lubricating oil in the reservoir 106 is cooled by cooling water flowing in a cooling water path 107 provided under the reservoir 106.
The lubrication of the thrust surface between the fixed ring 103 and the rotary ring 104 of the bearing is carried out by a splasher 105 which is attached to and driven through the driven shaft 102 and splashes the periphery of the thrust surface with lubricating oil. Nevertheless, it is difficult to obtain a necessary and sufficient amount of lubrication by an operation of a splasher, and thus an incomplete lubrication of the thrust plane of a mechanical seal occurs, and accordingly, a leakage of the atmosphere into the pump may occur.
Since the drive shaft 101 is directly connected to the rotors 108 of the pump, the temperature of which is elevated due to the compression heat during operation, the temperature of the drive shaft 101 is also elevated due to a heat conduction, and further, the temperature of the rotary ring 104 of the bearing installed on the drive shaft 101 becomes relatively high. Thus, because of a compression heat, a deformation of the thrust surface occurs between the rotary ring 104 and the fixed ring 103, which may allow a leakage of the atmosphere into the pump.
The drive shaft 101 directly connected with the rotors 108 of the pump is subjected to a vibration generated when the rotors transfer and compress a gas, and such vibration adversely affects the formation of an oil film on the thrust surface between the rotary ring 104 and the fixed ring 103 of the bearing, and thus causes a problem in that a leakage of the atmosphere into the pump may occur.
Also, since the drive shaft 101 is directly connected with the rotors 108, the rotational speed of the drive shaft 101 is selected on the basis of the rotational speed of the rotors necessary for maintaining the performance of the pump, and therefore, a disadvantage arises in that the circumferential speed of the thrust surface between the fixed ring 103 of the bearing and the rotary ring 104 of the bearing rotating together with the drive shaft 101 cannot be optimized.
To eliminate the problems described above, the realization of a pump having a high operation performance without a leakage of the atmosphere into the pump, and in particular, when pumping an inflammable or corrosive gas, an improvement of the safety and corrosion resistance of the pump is strongly desired.